update REFERENCE.md

REFERENCE.md

@@ -1605,6 +1605,7 @@ The following parameters are available in the `redis::instance` defined type:
 * [`cluster_slave_validity_factor`](#cluster_slave_validity_factor)
 * [`cluster_require_full_coverage`](#cluster_require_full_coverage)
 * [`cluster_migration_barrier`](#cluster_migration_barrier)
+* [`custom_options`](#custom_options)
 * [`output_buffer_limit_slave`](#output_buffer_limit_slave)
 * [`output_buffer_limit_pubsub`](#output_buffer_limit_pubsub)
 * [`managed_by_cluster_manager`](#managed_by_cluster_manager)
@@ -2398,6 +2399,14 @@ set if cluster_enabled is true
 
 Default value: `$redis::cluster_migration_barrier`
 
+##### <a name="custom_options"></a>`custom_options`
+
+Data type: `Hash[String[1],Variant[String[1], Integer]]`
+
+hash of custom options, not available as direct parameter.
+
+Default value: `{}`
+
 ##### <a name="output_buffer_limit_slave"></a>`output_buffer_limit_slave`
 
 Data type: `String[1]`